This article asks why cohabitants do not marry, when it would seem to be advantageous for them to do so. First, we examine the influence of the 'common law marriage myth', the mistaken but widespread belief that cohabitants gain marriage-like rights through living together for a certain time. Finding that this does not provide an adequate explanation for the social prevalence and acceptance of cohabitation, we then examine alternative rationalities, looking in turn at commitment in cohabiting partnerships, and how cohabitants use 'lived law' to create a DIY variety of marriage, as well as an alternative to it.